1. Field of the Invention
This invention relates to an antiskid brake system for a vehicle which controls brake fluid pressure so that an excessively large braking force is not applied to the wheels of the vehicle during braking, and more particularly to an antiskid brake system in which the brake fluid pressure is periodically increased and reduced according to the wheel speeds detected by wheel speed sensors.
1. Description of the Prior Art
There has been known an antiskid brake system which prevents the wheels from locking during braking, thereby preventing skid of the wheels. As disclosed, for instance, in Japanese Patent Publication No. 57(1982)-4544, the antiskid brake system basically comprises an electromagnetic control valve which is provided in the brake fluid system and controls the brake fluid pressure, wheel speed sensors which detect the rotational speeds of the wheels, and a control unit which controls the electromagnetic control valve to reduce the brake fluid pressure when locking of the wheels is detected through the wheel speeds. For example, the control unit calculates acceleration or deceleration of the wheels on the basis of the wheel speeds, calculates slip ratios of the wheels on the basis of the wheel speeds and a pseudo-vehicle-speed of the vehicle calculated on the basis of the wheel speeds, and causes the electromagnetic control valve to reduce the brake fluid pressure when the deceleration of the wheels becomes small than predetermined deceleration or when the slip ratios of the wheels reach a predetermined value while causes the electromagnetic valve to increase the brake fluid pressure when the rotational speeds of the wheels increases and the acceleration of the wheel speeds reaches a predetermined value after reduction of the brake fluid pressure. The control unit repeats such a control (will be referred to as "the antiskid control", hereinbelow) until the vehicle stops. The antiskid brake system prevents locking or skid of the wheels during hard braking, thereby stopping the vehicle in a short distance without the vehicle losing directional stability.
The wheel speed sensor which is employed in the antiskid brake system to detect the wheel speed measures the angle over which the wheel rotates per unit time. This kind of wheel speed sensor has a drawback that the measuring accuracy deteriorates as the rotational speed of the wheel lowers, and accordingly, the vehicle speed at a very low speed immediately before stop cannot be accurately estimated and the brake system provided with the antiskid brake system can operate just as a normal brake system without antiskid brake system at the worst.
On normal road having a relatively large surface friction coefficient, this problem may be overcome by interrupting the antiskid control when the wheel speeds lower to a predetermined value. That is, when the surface friction coefficient is large, the road surface gripping force of the wheels is strong and the wheels do not lock or skid even if a braking pressure proportional to the brake pedal pressing pressure directly acts on the brake fluid system.
However on road having small surface friction coefficient such as frozen road, the road surface gripping force of the wheels is poor, and the wheels are apt to lock or skid when the braking pressure sharply increases, whereby stability of the vehicle is lost.